Secondary Tie on Transformer
- Thread starter KRG9729
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
You need to comply with the rules in 450.3 for the transformer protection and 240.21(C) for the protection of the secondary conductors.
What we are trying to do is tap off a 480 volt primary, 120/208 volt secondary transformer that is already feeding a 200 amp MB panel. The transformer has a 100 amp primary side disconnect. We want to install a 100 amp main breaker panel directly from the secondary lugs to a 100 amp MB panel. What confuses me is the 125% rule when dealing with 600 volts or less. Is this combined or individually calculated. Thank you.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I believe many people determine transformer protection in the wrong order. The location of the secondary protection is not a concern of 450.3 therefore 240.21(C) applies which is why I recommend starting there.
First protect the conductors feeding the load, be it an appliance, a motor, or a panelboard. This requires you to follow 240.21(C). Once you have decided the size and the location of the secondary conductor protection you can move on to the transformer.
The transformer 450.3 rules are very simply:
1) If you protect the transformer primary at not more than 125% you do not care about the transformer secondary protection.
2) If you want to protect the primary at more than 125% you need to have secondary protection of 125% max.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
It would seem that 240.21(C)(1) indicates that a wye connected secondary cannot be protected by a primary only CB.
Then are can't add up the secondary CB's to be more than a single sized CB.
Then are can't add up the secondary CB's to be more than a single sized CB.
- Location
- Illinois
- Occupation
- retired electrician
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
It depends on the facility. 240.21(C)(3)(2) allows you to 'combine' secondary devices, in an industrial facility. The other tap rules allow you to have multiple devices as long as you run directly from the transformer terminals to a single protective device.
Table 450.3(B) Note 2; specifically allows the secondary protection to be the sum of up to 6 devices.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Don,
IMHO, 240.21(C)(1) indicates that the secondary conductors from a multiphase transformer cannot be soley protected by the primary of the transformer. So that means that secondary protection is required.
Jim,
I agree that note 2 allows multiple ocpd's but their summed total of the ratings cannot exceed that of a single device if it were provided.
IMHO, 240.21(C)(1) indicates that the secondary conductors from a multiphase transformer cannot be soley protected by the primary of the transformer. So that means that secondary protection is required.
Jim,
I agree that note 2 allows multiple ocpd's but their summed total of the ratings cannot exceed that of a single device if it were provided.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
450.3 is not about protecting the transformer secondary, it is about protecting the primary.
The OP did not mention what size transformer they have nor the conductor lengths, so it is hard to give an absolute answer.
The concept of one transformer feeding two different panels may be compliant under 240.21(C)2 and 240.21(C)5 as long as there is a single device at the end of each conductor/circuit. 240.21(C)(3) allows multiple devices at the end of the conductors.
The OP is using a 100A and a 200A device so the combined protection is 300A.
Now goto the transformer.
450.3(B) says the transformer needs to be protected on the primary by 125% maximum. If the OP's 100A device meets this requirement (which it would for a 75kVA or larger 480V primary transformer), then no further investigation is needed and the installation meets the NEC.
However, if the 100A device is larger than 125% and not larger than 250%of the primary (which would be the case for a 30-45kVA transformer), the combined secondary protection could not be larger than 250% of the secondary current (100A or 150A respectively), which means the installation would not be "per code".
- Location
- Illinois
- Occupation
- retired electrician
- Location
- Illinois
- Occupation
- retired electrician
It is my opinion that all of the required protection for the transformer, both primary and secondary, is found in Article 450. The rules in 240 only apply to the protection of the conductors.
It is my opinion that the word "transformer" includes both the primary and secondary windings.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I agree, 450 is about protecting the transformer windings not the conductors connected to them.
I apologize for my poor choice of wording.
Thank you for the replies...The transformer is a 75 KVA, 480 Delta Primary/208Wye Secondary. The existing 200 amp MB panel is approximately 7' of conductor length from the transformer. The new panel we are installing is about 22' conductor length from the transformer. We are back feeding the panel with a 3 pole 100 amp breaker. The #2 conductors will terminate at the transformer on new double barrel lugs along with the 2/0's from the 200 amp panel. From everything I read this should meet NEC requirements, please correct me if I'm missing something. Thank you again for the help.
- Status